The Flexible Coal Plant – How Some Coal Plants Are Transitioning to Peak Load
Due to new economic and political priorities, increasing interest and investment in renewable energy, growth in distributed energy, and cheap natural gas, power systems are being forced to adapt. Reduction in the share of electricity generated by coal is occurring and it is expected to continue. How can baseload coal plants thrive when they are not always in the money?
One North American coal plant, generically referenced as Coal Generating Station (CGS), has managed to leverage procedure and plant modifications to offer peaking power instead of baseload through more aggressive cycling. Designed to operate at 80% capacity, CGS now often runs at an output below 40% and cycles on and off as many as four times a day.
Coal-Fired Generation Landscape
- As of 2009, more than 60% of North American coal-fired generation was from units produced in the 1960s and 70s for baseload, which typically see only a few cold starts per year
- Since 1980, more efficient, large-capacity units have been built with efficient supercritical steam conditions but also are designed for baseload operation
- CGS coal units were designed for baseload but now average 1,760 starts per year as a peaking plant
- Impacts from this type of cycling include thermal fatigue, stresses on components and turbine shells, wear on auxiliary equipment, and corrosion of parts
- These impacts can lead to equipment fatigue and failure, specifically in the boiler, resulting in increased operations and maintenance costs and demand for more extensive training, inspection, and evaluation programs
- Equivalent forced outage rates, a measure of a plant’s reliability, ranged from 9% to 33% between 2002 and 2012. Typical EFOR for a baseload coal plant is 6.4%
Operating Procedure Modifications
- Approximately 90% of cost savings came from adjustments to operating procedures
- CGS has limited much of the potential damage to major and minor components from cycling by controlling temperature fluctuations during plant start-up and shutdown through rigorous inspection programs
- To reinforce the skills needed to monitor the impacts of cycling, additional training programs have been introduced
- Specific examples of operating procedures that have been adjusted include forced cooling, monitoring economizer inlet headers, layup procedures pressure part management, temperature monitoring for turbine parts, water chemistry maintenance, environmental controls, breaker maintenance, and overall monitoring programs
- Physical modifications, specifically those that focus on improving drainage/thermal resiliency and reducing corrosion opportunities, are also required for plant cycling
- Part replacement is often done on a case-by-case basis, where wholesale power market opportunities in the coming year are analyzed (assessing peak capacity requirements, role of the thermal fleet, likely dispatch order of units, cost to reduce EFOR per unit) to justify the cost of part replacements and reducing the forced outage rate
- Physical modifications include improvements to the boiler, pulverizers, turbines, generator rotors, and condenser
Replicability and Other Costs
- Replicating the results from one coal plant to another can be difficult due to variabilities in physical plant distinctions, operating distinctions, and regulatory distinctions
- Costs can be difficult to estimate and isolate due to the long timeframe that improvements are made over to evolve to a more aggressive cycling schedule
- Costs, other than the modifications listed above, may include those associated with increased emissions rate. Specifically, emissions are impacted due to increased fuel use, reduced plant efficiency, and reduced effectiveness of pollution-control equipment
- Studies suggest that avoided emissions from wind and solar far outweigh emissions increases from coal plant cycling
- Increasing emphasis on renewables, demand response, and other emerging technologies is forcing an increased proportion of the power generation fleet to be flexible in some markets to stay competitive
- Replicating CGS’s flexibility depends on many factors, but other coal plants can leverage the decades of learned plant and procedure improvements to tolerate high cycling levels without detrimental damage, unacceptable loss of plant life or thermal efficiency, while staying competitive in peak power
- It is important that the appropriate economics exist to compensate for the costs of maintaining grid-balancing plants if fossil-fired capacity must be available to keep power supplies reliable
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